Black Sea

• 1. Introduction

  ›

  The Black Sea holds a secret beneath its surface: below a certain depth, nothing breathes. Over 90% of the deeper water volume is completely anoxic, starved of oxygen, and it has been that way for a very long time. That dead zone is why wooden ships that sank here centuries ago have survived intact, their timbers refusing to rot. One such wreck, the Byzantine vessel Sinop D, was found preserved in the anoxic layer off the coast of Sinop, Turkey, a ghost from another age.

  This body of water sits between Europe and Asia, bounded by Bulgaria, Georgia, Romania, Russia, Turkey, and Ukraine. Its drainage basin, fed by rivers including the Danube, the Dnieper, and the Dniester, reaches across portions of 24 countries. It covers 436,400 square kilometers and plunges to a maximum depth of 2,212 meters. And yet, for most of its history, outsiders found it terrifying enough to call it the Inhospitable Sea. What made it so forbidding? And how did a sea once feared as hostile become one of the most contested and consequential waterways on earth?

• 2. A Sea of Many Names

  ›

  The earliest name on record belongs to the Hattians and their conquerors, the Hittites, who called it the Sea of Zalpa, after the Hattic city situated near the estuary of the Marrassantiya River, the river now known as the Kizil Irmak.

  Greek sailors encountered the sea and absorbed a name already in circulation. The principal Greek name Pontos Axeinos is generally accepted as a rendering of the Iranian word axshaina, meaning dark colored. Ancient Greek voyagers connected it instead to the Greek word axeinos, meaning inhospitable, and so the name stuck as the Inhospitable Sea. The geographer Strabo recorded in his Geographica that the name was thought to reflect the sea’s difficult navigation and the hostile people who lived along its shores before Greek colonization.

  Because the name was considered an ill omen, Greek sailors eventually flipped it to its opposite: Euxeinos Pontos, the Hospitable Sea. Both names appear first in the poet Pindar, around 475 BC. The contradiction lived on for centuries: the hospitable name was used in everyday Greek, while the true inhospitable name was kept for mythological contexts.

  Other cultures gave the sea entirely different identities. The tenth-century Persian geography book Hudud al-‘Alam called it the Georgian Sea. The Greater Bundahishn, a Middle Persian Zoroastrian scripture, referred to it as Siyabun. A 1570 map in Abraham Ortelius’s Theatrum Orbis Terrarum labeled it Mar Maggior, meaning Great Sea. The name Black Sea itself, along with equivalents like Karadeniz and Chyornoye more, cannot be conclusively shown to predate the 13th century. One contested scholarly theory holds that black signified north in a Turkic system of color-coded cardinal directions, which would place the name’s origin with a people living between the northern and southern seas, pointing to the Achaemenids of 550-330 BC.

• 3. The Hidden Plumbing

  ›

  Water in the Black Sea does not simply sit still. Two layers of water flow through the Bosporus simultaneously and in opposite directions. Denser, more saline water from the Aegean creeps in along the bottom while lighter, fresher Black Sea water pours out near the surface. The outflow runs at about 16,000 cubic meters per second, the inflow at around 11,000 cubic meters per second.

  Beneath the surface, between roughly 50 and 100 meters deep, lies a halocline, a sharp boundary of saltiness that blocks the layers from mixing. Below that sits the Cold Intermediate Layer, a remnant of winter surface water that descends and stalls. Beneath it, a major density boundary at about 100-200 meters locks the deep water away from the surface permanently. Down there, salinity climbs to 22.3 practical salinity units and temperatures settle around 8.9 degrees Celsius, while bacterial decomposition of sinking organic material consumes all available oxygen.

  On the 1st of August 2010, scientists at the University of Leeds announced the discovery of an undersea river flowing along the seabed of the Black Sea, the first of its kind ever identified. The river is a channel of particularly saline water that spills through the Bosporus from the Mediterranean and then carves its way along the bottom, following the contours of the seafloor like a river on dry land.

  Surface circulation follows its own logic. A basin-wide current called the Rim Current traces the perimeter of the sea in a cyclonic pattern. Inside it, two smaller cyclonic gyres turn in the eastern and western halves of the basin. These gyres are organized in winter but break apart into eddies during summer and autumn. Each spring, the Batumi eddy forms in the southeastern corner of the sea.

• 4. The Deluge Question

  ›

  At the height of the last ice age, global sea levels stood more than 100 meters below where they are today. The Bosporus, at its shallowest point only 36 meters deep, and the Dardanelles, at 55 meters, would have been dry land. Evidence suggests the Black Sea basin sat much lower than today during the post-glacial period, possibly existing as a landlocked freshwater lake in its upper layers.

  William Ryan, Walter Pitman, and Petko Dimitrov put forward what became known as the Black Sea deluge hypothesis, a theory that around 5600 BC, seawater from the rising Mediterranean breached the Bosporus sill in a catastrophic flood, rapidly raising the level of the Black Sea. The New York Times published news of the hypothesis in December 1996, just before it appeared in an academic journal.

  The scientific community agrees that the broad sequence of events described in the hypothesis did take place. The debates concern how sudden the flood was, exactly when it occurred, and how large the rise in water level was. A separate question hangs over the hypothesis: whether this prehistoric inundation might be connected to flood myths from the ancient world. No consensus has been reached on that point.

  The geological record adds further layers. The Black Sea was carved out partly by the splitting of an ancient volcanic arc and the subduction of the Paleo- and Neo-Tethys oceans, though the exact timing of those events remains uncertain. The collision of the Eurasian and African plates and the westward movement of the Anatolian block along the North Anatolian and East Anatolian faults still shapes the basin today, producing enhanced subsidence and significant volcanic activity in the Anatolian region.

• 5. Crossroads of Civilizations

  ›

  Greek presence along the Black Sea coast began at least as early as the 9th century BC, drawn by the grain grown in the hinterland. By 500 BC, permanent Greek communities existed all the way around the sea, and a trade network connected them to the wider Mediterranean. Over time these settlements developed their own distinct identity, called Pontic, separating from the cultures of the founding city-states that had sent them.

  The land at the eastern end of the sea, Colchis, in what is now Georgia, marked for the ancient Greeks the very edge of the known world. The Pontic-Caspian steppe to the north is seen by several researchers as the prehistoric homeland of the speakers of the Proto-Indo-European language.

  The sea was sailed by an extraordinary list of peoples: Hittites, Carians, Colchians, Armenians, Thracians, Greeks, Persians, Cimmerians, Scythians, Romans, Byzantines, Goths, Huns, Avars, Slavs, Varangians, Crusaders, Venetians, Genoese, Georgians, Bulgarians, Tatars, and Ottomans. The Republic of Genoa held control of the Crimean Peninsula until 1479. Within five years of losing it, the Ottoman Navy had made the Black Sea effectively its own lake. After that, the only Western merchant ships permitted to sail it were those of Venice’s old rival Ragusa. The sea also served as a trade route for enslaved people, moving them between Crimea and Ottoman Anatolia through the Crimean-Nogai slave raids in Eastern Europe.

  Imperial Russia entered as a Black Sea power in the late 18th century, occupying the littoral of Novorossiya in 1764 and Crimea in 1783. The Russian Black Sea Fleet, founded that same year, challenged Ottoman restrictions on navigation. The Crimean War, fought between 1853 and 1856, saw French and British forces clash with the army of Tsar Nicholas I. On the 2nd of March 1855, Nicholas I died and Alexander II took the throne. On the 15th of January 1856, Alexander II ended the war on terms that stripped Russia of its naval fleet on the Black Sea and turned the sea into a demilitarized zone.

• 6. The Ecology Under Pressure

  ›

  The Black Sea’s marine life has always been shaped by the anoxic deep, which acts as a sink for nutrients and prevents them from reaching the sunlit upper waters where life depends on them. The phytoplankton that thrive in those upper layers cycle through a distinctive annual rhythm: diatoms and dinoflagellates surge in spring, a weaker mixed community persists below the thermocline in summer, and production intensifies again at the surface in autumn. The coccolithophore Emiliania huxleyi blooms in late spring and summer, and is also responsible for releasing dimethyl sulfide into the atmosphere.

  Since the 1960s, rapid industrial expansion along the coastline and the construction of a major dam on the Danube pushed the sea into serious ecological trouble. The nitrogen-to-phosphorus-to-silicon ratio in the basin shifted, and the frequency of single-species algal blooms climbed sharply: diatom blooms increased by a factor of 2.5, and non-diatom blooms increased by a factor of 6. Benthic plants were starved of light, and anoxia caused mass die-offs in marine animals. Overfishing through the 1970s made the situation worse.

  In the late 1980s an invasive species, the warty comb jelly Mnemiopsis leidyi, arrived in the basin. Starting from just a few individuals, it grew to an estimated biomass of one billion metric tons. It consumed copepods and other zooplankton that the rest of the food web depended on.

  Mediterranean monk seals, once abundant here, were regarded as having gone extinct from the Black Sea basin in 1997. Monk seals had still been present at Snake Island, near the Danube Delta, until the 1950s. Pollution-reduction efforts and new sewage-treatment plants built in Slovakia, Hungary, Romania, and Bulgaria during the 1990s in connection with European Union membership helped the ecosystem begin to recover, and a monitoring program called EROS21 found decreased nitrogen and phosphorus levels relative to the 1989 peak.

• 7. War and the Contested Sea

  ›

  Sevastopol was one of the strongest fortified naval bases in the world when German forces attacked it during the Siege of Sevastopol from 1941 to 1942. The city sat on a bare limestone promontory at the southwestern tip of Crimea, with deeply eroded cliffs above Severnaya Bay making amphibious assault nearly as dangerous as a land approach. Its coastal batteries held 180mm and 305mm re-purposed battleship guns, shielded by reinforced concrete and armored turrets 9.8 inches thick. German forces succeeded in taking it, assisted by the massive rail gun known as Schwerer Gustav.

  On the 7th of November 1941, a Soviet hospital ship was sunk by German aircraft while evacuating civilians and wounded soldiers from Crimea. An estimated 5,000 to 7,000 people died in the sinking, with only eight survivors, making it one of the deadliest maritime disasters in history.

  The 1936 Montreux Convention established the framework that still governs the straits. It permits free passage for civilian ships but places strict limits on military vessels from countries that do not border the Black Sea. The 1982 amendments gave Turkey the power to close the straits at its discretion in both war and peace.

  On the 24th of February 2022, two Russian navy warships attacked and captured Snake Island. On the 14th of April 2022, Ukrainian missiles sank the Moskva, the flagship of the Russian Black Sea Fleet and the largest ship lost in naval combat in Europe since World War II. Ukraine announced it had driven Russian forces off Snake Island on the 30th of June 2022. On the 14th of March 2023, a Russian Su-27 intercepted and damaged an American MQ-9 Reaper drone, which then crashed into the Black Sea. As of January 2025, Estonian Navy Commander Ivo Vark described the entire sea as a contested maritime area where both sides maintain room for action, with gas drilling platforms equipped with surveillance devices frequently changing hands between the two sides.